Although Drosophila is being used successfully to predict carcinogenicity of chemicals, some potent carcinogens were negative in this system. The apparent insensitivity of Drosophila may, however, be due to an inappropriate end point chosen to test a particular suspect carcinogen. Therefore, some potent carcinogens which did not respond to the most sensitive sex-linked recessive lethal test, will be tested for their capacity to induce mosaic mutations. If a majority of these negative carcinogens do induce mosaics, this test can be used for screening such compounds for their carcinogenicity. These studies will be supplemented with experiments designed to understand the origin of complete mutations be investigating the relative proportion of complete and mosaic mutations induced by different classes of chemicals. Both repair deficient and repair proficient strains and various classes of chemicals will be utilized in these studies. Pedigree analyses will be extended for one more generation to understand the nature and mechanism of induction of replicating instabilities which is one of the most intriguing problems of modern mutation research.